Sponsored by: Jt. SMD/MSD Nuclear Materials and MSD Flow and Fracture Committees and FEMS (Federation of European Materials Societies)
Program Organizers: R.J. Arsenault, Deptartment of Materials Science and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115; David Cole, CRREL, 72 Lyme Rd., Hanover, NH 03755; Todd Gross, Department of Mechanical Engineering, University of New Hampshire, Durham, NH 03824; Gernot Kostorz, Institut für Angewandte Physik, ETH Hönggerberg, CH-8093 Zürich, Switzerland; Peter Liaw, Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996-2200; Sivan Parameswaran, NRC-Institute for Aerospace Research, Ottawa, Canada K1A 0R6; Howard Sizek, Inco Alloys International Inc., Huntington, WV 25705-1771
Wednesday, AM Room: Orange County 3
February 7, 1996 Location: Anaheim Marriott Hotel
Session Chairpersons: K.A. Heck, Inco Alloys International, Inc., 3200 Riverside Dr., Huntington, WV 25705-1771. T.G. Langdon, Department of Material Science, University of Southern California, Los Angeles, CA 90089-0241
8:30 am Invited
DISLOCATION STRUCTURE EVOLUTION AND DEFORMATION KINETICS OF PURE MATERIALS: W. Blum, Institut für Werkstoffwissenschaften LS I, Universität Erlangen-Nürnberg, Martensstraße 5, D-91058 Erlangen, Germany
Results from work hardening tests at constant strain rate and from creep tests at constant stress [[sigma]] for aluminum and other pure materials are combined to discuss the mechanical equation of state relating ,[[sigma]] and structure at constant temperature. The dislocation structure is mainly determined by [[sigma]]. Subgrain boundaries play an important role as hard regions and as dislocation sinks. Despite the complications related with subgrain and grain boundaries, Weertman's natural law of steady state deformation is a good approximation of the steady state -[[sigma]] relation in the limit of low stresses. At high stresses the interaction between climb and glide must be taken into account. Information on glide kinetics is available from tests at constant structure ( -change, [[sigma]]-change and [[sigma]]-relaxation tests). The relative decrease of the apparent activation area of glide, which is found as the steady state is approached, is interpreted as a result of concurrent recovery.
9:00 am Invited
CREEP MECHANISMS IN DISPERSION ON STRENGTHENED MATERIALS: A.K. Mukherjee, R.S. Mishra, Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616
The steady state creep mechanisms in dispersion strengthened materials are reviewed. The influence of microstructural parameters like interparticle spacing and grain size on the steady state dislocation creep are highlighted. The analysis shows that the steady state dislocation creep mechanism depends on the ratio of interparticle spacing and grain size. In materials with fine particles, the individual dislocation-particle interaction creep dominates. The dislocation creep data on Ni-ThO2 and Al-Al2O3-Al4C3 alloys shows a grain size dependence. A rationale is developed to explain this behavior. A comparison of the kinetics constant shows that the creep kinetics is slower in dispersion strengthened materials even after compensating for the threshold stress. This is explained on the basis of geometrical details of dislocation climb bypass of particles. On the other hand, the dominant steady state dislocation creep mechanism in several aluminum alloy composites, where the particles are quite coarse, is constant substructure creep. The possible reasons for the variation of true stress exponent for aluminum alloy composites from a value of 5 to 8 are discussed.
TORSIONAL BEHAVIOR AND DAMAGE MECHANISMS IN Ti-6Al-4V ALLOY AND TiC/Ti-6Al-4V COMPOSITE: J.H. Zhu, P.K. Liaw, Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200; J.M. Corum, M. Ruggles, Engineering Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831
The 1000deg.F torsional and creep properties under torsional loading of both baseline Ti-6Al-4Al and Ti-6Al-4V reinforced with 10 wt%TiC particulate were investigated in this paper. It has been found that the torsional yield strength, the modulus of elasticity in shear, the modulus of rupture, and torsional creep resistance of the composite are greater than the unreinforced matrix alloy at the test temperature, whilst the ductility of the composite is much lower than that of the matrix alloy. A replication technique was used to monitor the microstructural change and damage development during the various stages of testing, which was assisted by scanning electron microscopy. Furthermore, analytical transmission electron microscopy was used to assess the nature and extent of the interactions between TiC particulate and matrix of the Ti-6Al-4V/TiC composite during the torsional creep testing.
A MODEL FOR DISLOCATION CREEP FOR FINE GRAIN METALS: A.K. Ghosh, Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136
Grain boundary sliding has been known to influence creep behavior in metals deforming primarily by dislocation glide-climb process. It has been realized in recent times that grain boundary sliding itself results from localized glide-climb creep in the vicinity of the grain boundaries, i.e., the grain mantle region. In a recent model, we have proposed that the number of dislocation sources activated at grain boundaries depends on the strength of obstacles and the applied stress. At low stresses, the emission and absorption of dislocations are confined to the grain boundary region. At large stresses, dislocation creep of the grain core becomes dominant, but due to the relative ease of grain boundary sliding, stress concentration at triple points also builds up. The resulting acceleration of creep in the grain corner regions provides an additive creep component in the conventional power law creep regime. This model will be discussed and illustrated with data on metals and intermetallics. Research sponsored by AFOSR-URI Grant No. F 49620-93-1-0289 and DoEER program.
DEFORMATION MODES OF THE SUPERPLASTIC AL-10wt% Ti ALLOY AT HIGH STRAIN-RATES: Dong-Wha Kum, Hye-Sung Kim, Sang-Hee Suh, Korea Institute of Science and Technology, Cheongryang P.O. Box 131, Seoul 130-650, Korea; Won-Tac Kim, Department of Physics, Chongju University, Naedok-dong 36, Chongju 360-764, Korea
Gas-atomized Al-10wt% Ti powders were consolidated by power metallurgy process involving hot extrusion, and two-phase microstructure consisted of micro-size aluminum grains and fine flake-shaped Al3Ti particles was obtained. The fine-grained Al-10wt% Ti alloy showed very high ductility at strain-rate of 0.1/sec at temperatures above 600deg.C. This high strain-rate superplastic(HSRS) behavior is correlated with that of whisker-reinforced aluminum composites processed by powder metallurgy, referring to the microstructural similarities. Because local melting did not occur in the Al-Ti binary system at the test temperatures, requirement of incipient melting of HSRS is questioned. The HSRS behavior is interpreted by an extension of the microstructural superplasticity which has been well characterized in fine-grained metallic alloys.
10:30 am BREAK
EFFECT OF GRAIN SIZE IN POWER-LAW CREEP: K. Sadananda, C.R. Feng, Code 6323, Naval Research Laboratory, Washington D.C. 20375
Grain size effects are generally observed during Newtonian viscous flow with stress exponents equal to one, but not during power-law creep, where the exponents are of the order of 3 to 5. Our recent studies of creep of molybdenum disilicides and their composites showed significant grain size effects even during power-law creep, and these effects are strain-history dependent. We examine these effects to evaluate the generality and factors that govern the behavior.
THE SIGNIFICANCE OF HARPER-DORN CREEP IN METALS, CERAMICS AND GEOLOGICAL MATERIALS: M. Zhou, Department of Materials Science, University of Southern California, Los Angeles, CA 90089-0241; D.M. Owen, Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, CA 91125; T.G. Langdon, Department of Materials Science and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453
Harper-Dorn creep is now established as an important deformation mechanism which occurs at low stresses with a stress exponent, n, of 1. Recently the possible significance of Harper-Dorn creep in a range of f.c.c. and b.c.c. metals. This paper examines this controversy and further considers the possible role of Harper-Dorn creep in ceramic and geological materials.
INDENTATION CREEP: O. Prakash, Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4M7
The issues concerning the analysis of indentation creep data are discussed. A model for analyzing indentation creep data in terms of back/internal stresses resulting from the interaction of mobile dislocations with microstructural inhomogeneities is proposed. The model enables us to rationalize creep indentation data to uniaxial creep results for a range of materials including metals and metal-type organic crystals and their particulate composites. The effect of recovery in indentation creep is considered, and it is shown that under conditions of slow recovery, indentation creep can be considered similar to the creep of a "constant structure" material subjected to a continuously decreasing applied stress. At high homologous temperatures, stress exponents close to 3 are obtained in agreement with the natural power law theories of creep deformation first proposed by Professor Weertman.
A MECHANISTIC MODEL FOR TRANSIENT SUPERPLASTIC DEFORMATION: K. Zhang, C.H. Hamilton and H.M. Zbib, Department of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920
The primary focus of research on superplasticity has been on the
characterization and modeling under deformation conditions considered to be
"steady-state", as a result, many models have been developed for the
superplastic flow and constitutive relations have been proposed to represent
the mechanistic characteristics of the relationship among flow stress, grain
size, diffusion parameters, and other physical parameters. The general form of
the mechanistic models has been used in formulating reasonable constitutive
relations for superplastic forming (SPF) process. In earlier investigation, a
strong transient behavior has been found in superplastic deformation of model
material Pb-Sn eutectics, and results in difference in the stress/strain rate
relation under loading and unloading(relaxation), both of which are different
from the steady-state relationship. In this study, a mechanistic model, with
consideration of internal stress in superplastic deformation, will be proposed
to account for the transient effects with superplastic deformation. The
theoretic calculation based on this model shows good representation of
experimental results with Pb-Sn eutectic and engineering materials. The
mechanism with the transient superplastic deformation will be considered.
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